Pick your plan has an unlimited data option. Pay as you go only has 1mb or 5mb data options ($5 and $10 respectively). Pick your plan, however, has to be tied with a credit/debit card on auto-pay and it is taken out at the beginning of your month, depending on the day you sign up. Pick your plan has the 1mb, 5mb options as well as 10mb for $15 and unlimited for $20.
You can do pay-as-you-go data on either option, but trust me from experience, it goes quickly just from little data pulls here and there (weather, email, not sure what else but my cents would just randomly tick away). Wouldn't do the pay-as-you-go option unless you're willing to lose your latest refill quickly.